The present invention relates to a suspension for vehicles, in particular road vehicles. It relates both to the guidance of a vehicle wheel relative to the body of this vehicle and to the organization of the deflection of the wheel relative to the chassis. It relates more particularly to means used for actively controlling the position of the wheel plane relative to the body.
It is known that a wheel is mounted on a hub and that the hub is mounted on a wheel carrier by means of a rolling bearing which embodies the axis of rotation of the wheel. The guidance of the wheel involves allowing the wheel carrier to be displaced relative to the vehicle by an extent which is sufficient for the suspension of the vehicle. Moreover, the guidance of the wheel must be such that the control of the path of the vehicle via the grip of the tires on the ground is ensured under the best possible conditions. The deflecting movement which is sought is of great extent in the vertical direction.
It is customary for this movement to be designated by the expression xe2x80x9cvertical suspensionxe2x80x9d. It may be desirable to allow slight movement in the longitudinal direction (xe2x80x9chorizontal suspensionxe2x80x9d is then referred to), while, ideally, the aim is to avoid any relative movement in the transverse direction. Moreover, the steering and the camber variations which all these movements may induce must be strictly contained, thus also resulting in a design constraint which the average person skilled in the art must take into account.
If the xe2x80x9cplanexe2x80x9d of the wheel is what is meant by the plane perpendicular to the axis of rotation of the wheel and passing through the middle of the wheel or, more specifically, through the center of the area of contact with the ground of a tire mounted on said wheel, the control of the deflecting movements is tantamount to saying that guidance must ensure a strictly controlled position of the plane of the wheel relative to the chassis. It is still possible to define a longitudinal and vertical plane forming a reference to identify the preferred running direction which is parallel to said longitudinal plane. The purpose of guiding the plane of the wheel is to control as strictly as possible the relative position of the plane of the wheel relative to said longitudinal plane in terms of angle and distance.
The guidance of the wheel plane directly affects the attitude of the tire in relation to the ground, hence the stresses which said tire undergoes and the more or less favorable position which it assumes in order to transmit forces, in particular forces in the transverse direction. Now these forces are known to be of primary importance for the guidance of the vehicle and therefore for safety.
Road vehicles in use at the present time are known to experience a rolling movement precisely because of their suspension. In fact, the centrifugal force causes a transfer of load toward the outside of the bend, thus resulting in an increase in compression of the suspensions on the outside of the bend and an expansion of the suspensions on the inside of the bend. Now this roll is detrimental both to passenger comfort and to the roadholding of the vehicle due to the increase in the camber of the wheels which it brings about. The means used most often to combat this roll involves using one or more antiroll bars. Unfortunately, an antiroll bar can only limit the roll by opposing a reaction torque to the latter. In principle, it cannot prevent roll from taking hold, and therefore it cannot prevent the wheels from being inclined in a direction unfavorable to the proper functioning of the tires.
Other solutions making it possible to combat roll more radically are also known. It is possible to conceive a pendulum suspension (purely passive). Such suspensions can force the wheel plane to be inclined toward the inside of the bend by means of a suitable mechanism, or else they can also transfer the rolling axis of the vehicle above the center of gravity, so that the vehicle body is naturally inclined toward the inside of the bend. However, pendulum suspensions have not had much success, probably because of the difficulty of installing suspension arms and joints making it possible to achieve this result. The intrusion of suspension arms and joints in places which it is desirable to leave free for the passenger compartment or for installing the engine or transmission of the vehicle would appear to be a disadvantage. This problem seems to limit this type of suspension to a few types of vehicle, such as, for example, in the field of railroads. Moreover, in general terms pendulum suspensions increase the overload on the wheels located on the outside of the bend by displacing the center of gravity of the vehicle toward the outside of the bend. This is particularly detrimental with regard to road vehicles equipped with tires.
Moreover, research for an even better compromise between suspension comfort and roadholding leads to the provision of so-called active or semiactive suspensions, the control of which is now made possible by advances in electronics. However, it is found that these control means are transplanted onto suspensions which are still of traditional mechanical design, said control means being adapted to the defects of these suspensions, without looking to make use of new parameters in suspension kinematics. In other words, active or semiactive suspensions are concerned simply with controlling the damping characteristics of the suspensions and even the flexibility characteristics, while at the same time preserving suspension kinematics derived directly from conventional forms of construction.
The object of the invention is to improve the operating safety of vehicles by means of suspension kinematics allowing active control of the camber of the wheel, in order to maintain the tires in a position relative to the ground which is as favorable as possible to their grip and their fatigue strength, even in the event of extremely severe stresses.
The invention provides a vehicle comprising:
a suspended running chassis making it possible to define thereon a longitudinal and vertical plane forming a reference, the preferred running direction of the vehicle being parallel to said longitudinal plane,
at least two wheel connecting systems, each comprising a wheel, said wheel connecting systems being mounted on said chassis on either side of the latter in a transverse direction, each of the wheel connecting systems comprising a wheel mounted on a support by means of a suspension device allowing deflection of the wheel relative to the support, said suspensions being independent of one another, the deflecting movement taking place in the plane of the wheel, the deflection stroke being sufficient to afford the required vertical suspension movement,
wherein each support is mounted on said chassis by means of a camber mechanism making it possible to vary the camber angle of the corresponding wheel, said camber mechanism acting between the chassis and each support, so as to incline the plane of each wheel relative to said longitudinal plane in order to impart a camber angle of desired amplitude to the planes of said wheels, thus inclining all the wheels relative to the ground in the same direction.
It is especially advantageous to control the camber angle so as to improve the behavior or comfort of the vehicle on a bend. The invention makes it possible to incline the plane of each wheel in the opposite direction to transverse acceleration. For this purpose, the vehicle comprises at least one sensor for recording the value of at least one parameter making it possible to evaluate the transverse acceleration exerted on the vehicle, and calculation means for calculating the amplitude of each camber angle as a function of the value or values recorded, said camber mechanism comprising control means acting under the control of said calculation means.
Typically, the transverse acceleration in question is the centrifugal force occurring on a bend. The invention allows active control of the camber of the wheels. By virtue of the arrangement provided by the invention, a tendency of tires to exhibit pronounced wear at the shoulders is combated effectively, this tendency being well known in passenger cars driven for sport. In this case, the tread may be excessively offset axially toward the center of the vehicle. The active control of the camber corrects this tendency. Moreover, even when, simultaneously, the wheel experiences camber variations and the vertical suspension experiences pronounced deflections, the mode of functioning of the tire remains as far as possible optimal.
It may be noted that the camber is the angle which the plane of the wheel forms relative to a straight line perpendicular to the ground. Of course, it is not possible to act directly thereon by controlling the position of the wheel relative to the ground, since it is inconceivable to come to bear on the ground itself in order to control the camber variation. It is possible only to act indirectly via a mechanism connected to the chassis.
The invention applies particularly to four-wheeled vehicles, such as passenger cars. The camber angle variation concerned here is of great extent, that is to say is nearer the values frequently swept by motorcycles than the set values which may exist in a conventional four-wheeled passenger car. For clarification, the range of variation is of the order of xc2x115xc2x0 to xc2x120. At all events, the range of variation is such that, when the wheel tilts about its area of contact with the ground, the transverse displacement of the radially upper part of the wheel may reach an extent greater than the width of said wheel. Of course, the amplitude of the camber is possibly not identical for each of the wheels of the vehicle, whereas the tilting direction of the wheels is always the same and is directed opposite to the transverse acceleration originating from the centrifugal force. The top of the wheel therefore tilts in the centripetal direction.
It can be seen that the invention provides suspension kinematics which are as pure as possible: the wheel connecting system not only offers vertical suspension, but it also makes it possible to modify the camber of the wheel at any moment, without thereby reducing the stroke of the vertical suspension in terms of either compression or expansion, while at the same time maintaining vertical suspension under optimal operating conditions. Strictly speaking, it would be more appropriate to refer to deflection located in the wheelplane and directed perpendicularly to the running movement of the vehicle. For the sake of linguistic convenience, this degree of freedom is designated by xe2x80x9cvertical suspensionxe2x80x9d.
In an especially advantageous variant, the invention provides an assembly making it possible to integrate the guidance and the elements necessary for the so-called vertical suspension into the interior of the wheel. The essential functions of the suspension (guidance of the wheel plane, degree of freedom of deflection in the desired direction and control of the deflecting movements) are thereby integrated into the volume within the wheel. This makes it possible to ensure that the assembly is highly compact.
According to another variant, the invention provides a passenger car with active control of roll: the passenger compartment of the vehicle is mounted on the chassis in such a way that said compartment can be inclined toward the inside of the bend as a function of the centrifugal force. Here again, although roll is a measure of the lean angle of the passenger compartment relative to the ground, action is taken on the roll angle indirectly by bearing on the chassis. The object is to achieve a substantial range of variation of the roll angle of the passenger compartment, of the same order of magnitude as the range of variation of the camber. Thus, without necessarily achieving the roll angles which a vehicle having two in-line wheels (for example a motorcycle) may assume, the effect produced is sufficiently noticeable to alter radically the impression felt by the passengers of such a vehicle and thus to contribute to their comfort.
Another embodiment provides a horizontal suspension of the wheels, that is to say a suspension which allows the wheels to move along the longitudinal axis relative to the chassis when they encounter an obstacle. This contributes to comfort by avoiding transmitting violent shocks to the tire, due to the reduction in the impact speed.
The invention will be understood better from the following description relating to a four wheeled vehicle. The invention applies particularly to vehicles comprising at least four wheels, although the example given is not limiting.